1. Field of the Invention
The invention concerns a mounting assembly adaptable for use with either ceramic or metallic outlet guide vane airfoils and particularly relates to a cantilevered, spring loaded mounting assembly which may be preloaded to maintain a compressive force on the airfoils throughout all phases of engine operation and shut down.
2. Description of Prior Developments
With the present demand for higher performance turbine engines has come the need for higher engine operating temperatures. These higher operating temperatures, in the order of 2500.degree. F., have caused designers to turn to ceramic materials for replacing various turbine engine components previously fabricated from metals. Ceramic materials, however, pose their own design problems in that ceramics do not perform as well under tension as they do under compression. Moreover, ceramics generally expand thermally at a rate of approximately one fourth that of metals. This difference in thermal expansion, coupled with the need to maintain ceramics in compression poses a significant problem when ceramic and metal components are coupled or interconnected within a high temperature environment such as within a modern gas turbine engine.
That is, as a turbine engine approaches its operating temperature upon start up, the metal engine components expand more rapidly than the ceramic engine components. This differential expansion can cause the metal components to grow away from the ceramic components to which they are connected and place the ceramic components under an undesirable tensile load. Moreover, the large temperature changes during turbine engine thermal cycling can result in destructive thermal stresses within any metal or ceramic component which is constrained within a rigid structure.
The design considerations noted above are particularly applicable to the design of turbine engine outlet guide vanes which are typically arranged within the engine flow path for removing swirl from the hot exiting exhaust gasses. As the hot motive fluid leaves the turbine buckets of a conventional jet engine turbine, there is frequently present a swirl or tangential component of velocity caused by various factors including tip leakage over bucket ends, and by temperature and velocity stratifications in the fluid issuing from the combustion chamber. Elimination of the turbine exit swirl reduces the energy losses in the flow path downstream from the turbine exit. By redirecting the tangential velocity component into an axial velocity component, the outlet guide vanes add to the total useful thrust produced by the turbine engine.
The outlet guide vanes are usually arranged between inner and outer engine frame members such as the inner and outer casings. The airfoil profile of the outlet guide vanes channels the whirling vertical gasses into a plurality of evenly flowing axial streams which are conducive to optimum afterburner performance. The even axial flow aids in controlling the diffusion of the exiting fluid through the afterburner thereby promoting the full combustion of all oxygen present in the exhaust gasses.
Accordingly, a need exists for a mounting assembly for accommodating metal as well as ceramic outlet guide vanes within the flow path of a modern high temperature gas turbine engine. A further need exists for an outlet guide vane mounting assembly which maintains a compressive load on ceramic outlet guide vanes during all phases of engine operation and shut down. Moreover, a need exists for an outlet guide vane mounting assembly which minimize thermal stresses within the outlet guide vanes as they thermally expand and contract during engine operation.